This exercise from Biggest Loser trainer Jen Widerstrom may have a funny name, but it will seriously work your butt, thighs, and core. It's a variation on the fire hydrant, with an extra kick for good measure. Part of the challenge is bracing yourself so you don't sink to the side.
Here's how to do it: Start on all fours, with your back in a neutral position and your hands under shoulders and knees under hips. Lift your left knee from your hip out to the side, approaching parallel. Then kick your leg out, pointing your foot. Bring your foot back in, then lower your knee back to the floor.
Do 10 slow reps, followed by 10 fast reps. Repeat on the opposite side.